Horrible exporting on long shadow effect

Don't create the long shadow (Block shadow, Superhero) like that. Period.

Blends are not suitable for it. Neither is using the transform effect with gazillions of copies. 

There are better ways.

https://www.vektorgarten.de/long-shadows-with-illustrator.html

https://youtu.be/SD1HARFrszs 

The problem looks similar to moiré. Somehow the size of the graphic and number of steps in the blend are not working well with the target size and resolution of the PNG image you're creating.

You could try experimenting with the blend by varying its number of steps.

Color choices can also affect the look of the blend. Are you working in RGB or CMYK color mode? The black at the far end of the blend looks like a flat CMYK black that uses only black ink. That kind of a black has a more limited number of blend combinations than a deeper rich black that uses all four CMYK ink colors. A blend from Red to Rich Black is going to look more natural.

Hello @andrea_8933,

Would you mind sharing a link to a sample AI and screenshots of the export settings used after uploading them to a file-sharing service like Google Drive, so I can check it on my end?

Looking forward to hearing from you.

Anubhav